Observation of oxygen dimers via energy transfer from silicon nanoparticles

Energy transfer from photo-excited excitons confined in silicon nanoparticles to oxygen dimers adsorbed on the nanoparticle surfaces is studied as a function of temperature and magnetic field.</p

Porous silicon bulk acoustic wave resonator with integrated transducer

We report that porous silicon acoustic Bragg reflectors and AlN-based transducers can be successfully combined and processed in a commercial solidly mounted resonator production line. The resulting device takes advantage of the unique acoustic properties of porous silicon in order to form a monolithically integrated bulk acoustic wave resonator

Porous silicon bulk acoustic wave resonator with integrated transducer

We report that porous silicon acoustic Bragg reflectors and AlN-based transducers can be successfully combined and processed in a commercial solidly mounted resonator production line. The resulting device takes advantage of the unique acoustic properties of porous silicon in order to form a monolithically integrated bulk acoustic wave resonator

Synergy of nanocrystalline carbon nitride with Cu single atom catalyst leads to selective photocatalytic reduction of CO2 to methanol

Carbon nitride (C3N4) possesses both a band gap in the visible range and a low-lying conduction band potential, suitable for water splitting and CO2 reduction reactions (CO2RR). Yet, bulk C3N4 (b-C3N4) suffers from structural disorder leading to sluggish reaction kinetics. This can be improved by graphitisation; however, current processes in the literature, lead to a variety of graphitised C3N4 (g-C3N4), making it difficult to link the degrees of graphitisation with the functional properties. Herein, we employ complementary analyses, including electrochemical impedance, photoluminescence, and photocurrent, to elucidate structure–property–function relationships. Guided by the descriptors, we developed a facile two-step annealing method that yields nanocrystalline carbon nitride (nc-C3N4), comprising nanoscale graphitic domains within an amorphous matrix. The nanocrystalline grains of nc-C3N4 allow effective immobilisation of Cu atoms and stabilisation of low oxidation states (Cu(I)). Electron microscopy and energy-dispersive X-ray spectroscopy demonstrate that Cu is atomically dispersed. Importantly, the addition of only 0.11 wt% of copper to nc-C3N4 drastically decreases the charge recombination and resistance to change transfer. The synergy of the Cu single-atom catalyst and nanocrystalline domains in carbon nitride (Cu/nc-C3N4) leads to a remarkable 99% selectivity towards methanol production with a rate of 316 μmol gcat−1 h−1 during the photocatalytic CO2RR, which is absent in Cu/b-C3N4

Quasi-periodic Fibonacci and periodic one-dimensional hypersonic phononic crystals of porous silicon:Experiment and simulation

A one-dimensional Fibonacci phononic crystal and a distributed Bragg reflector were constructed from porous silicon. The structures had the same number of layers and similar acoustic impedance mismatch, and were electrochemically etched in highly boron doped silicon wafers. The thickness of the individual layers in the stacks was approximately 2 μm. Both types of hypersonic band gap structure were studied by direct measurement of the transmittance of longitudinal acoustic waves in the 0.1-2.6 GHz range. Acoustic band gaps deeper than 50 dB were detected in both structures. The experimental results were compared with model calculations employing the transfer matrix method. The acoustic properties of periodic and quasi-periodic structures in which half-wave retarding bi-layers do not consist of two quarter-wave retarding layers are discussed. The strong correlation between width and depth of gaps in the transmission spectra is demonstrated. The dominant mechanisms of acoustic losses in porous multilayer structures are discussed. The elastic constants remain proportional over our range of porosity, and hence, the Grüneisen parameter is constant. This simplifies the expression for the porosity dependence of the Akhiezer damping.</p

Porous Silicon As An Acoustic Material For Baw

Layers of porous silicon with a varying degree of porosity between 30% and 75% were prepared by electro-chemical etching from bulk-Si. Thickness of the layers was in a range of 2 to 4 μm. Acoustic impedance and velocity for longitudinal waves in a frequency range of 2GHz were evaluated utilizing a BAW resonator built on top of the porous Si layers. Material parameters were extracted from the electrical impedance characteristics of the BAW resonators. Special pre-processing of the frequency dependent impedance data was applied to simplify an initial fit to a Mason-Model. A subsequent fit to full data was performed to refine acoustic material parameters and estimate the propagation loss occurring in the porous Si. TEM cross-sections were prepared to verify the thickness of the porous Si layers. The longitudinal acoustic impedance was found to be in the range between 4.6 and 11.1 Mrayl while velocity was 4900 to 6950 m/s. Propagation loss was found to be lower than one would expect from a porous film. © 2012 IEEE

Photo-oxidation by singlet oxygen generated on nanoporous silicon in a LED-powered reactor

An annular flow photochemical reactor illuminated by UV and green (524 nm) light emitting diodes (LEDs) was characterised by a chemical actinometer. Very high efficiency of absorption of photons, most likely promoted by the specific orientation of LED elements in the reactor, was calculated based on the measured actinometry results. Generation of singlet oxygen mediated by nanoporous silicon under illumination by Ar+ laser, UV and green LEDs was demonstrated by indirect measurement of suppression of porous Si photoluminescence, and by direct measurements of singlet oxygen luminescence. The efficiency of reactor in singlet oxygen mediated reactions was tested using reaction of decomposition of diphenylbenzofuran. Estimated quantum yield of chemical reaction is about 34%

Chemical Kinetics of Metal Single Atom and Nanocluster Formation on Surfaces:An Example of Pt on Hexagonal Boron Nitride

The production of atomically dispersed metal catalysts remains a significant challenge in the field of heterogeneous catalysis due to coexistence with continuously packed sites such as nanoclusters and nanoparticles. This work presents a comprehensive guidance on how to increase the degree of atomization through a selection of appropriate experimental conditions and supports. It is based on a rigorous macro-kinetic theory that captures relevant competing processes of nucleation and formation of single atoms stabilized by point defects. The effects of metal-support interactions and deposition parameters on the resulting single atom to nanocluster ratio as well as the role of metal centers formed on point defects in the kinetics of nucleation have been established, thus paving the way to guided synthesis of single atom catalysts. The predictions are supported by experimental results on sputter deposition of Pt on exfoliated hexagonal boron nitride, as imaged by aberration-corrected scanning transmission electron microscopy